Conventional lubricating oil for diesel engines has been required to exhibit high detergency and antiwear, since it has been frequently used under the severe conditions. Therefore, such lubricating oil for diesel engines comprises, as a detergent-dispersant, sulfonate, phenate, salicylate and phosphonate of alkaline earth metal or alkaline metal, and overbased compounds thereof, which are metal based detergent-dispersants.
Further, zinc dialkyldithiophosphate (Zn-DTP) has been incorporated therein so as to keep antiwear thereof. Meanwhile, diesel engines have been utilized extensively due to their high power and good thermal efficiency. However, there has been an important issue for taking antipollution measures and prime task of reducing NOx (nitrogen oxides) and PM (particulate: Particulate Matter).
As one of those measures, it has been conducted for reducing NOx to lower a combustion peak temperature by heightening exhaust gas recirculation (EGR) ratio or retarding fuel-injection timing. However, if the combustion peak temperature is lowered, black smoke and PM increase, therefore it is required to install a device for an exhaust gas post-treatment.
As the exhaust gas post-treatment device, DPF (diesel particulate filter) for trapping PM or oxidation catalysts have been investigated. However, since both of them have filter structure, there have been the plugging problems thereof caused by metals in conventional diesel lubricating oils when the oils were used.
In addition, the reduction of the metal content, which means reduction of metal-based detergents and antiwear agents, causes deterioration of detergency and antiwear.
Therefore, there has been actively conducted development of lubricating oils for diesel engines having low ashes, high detergency and antiwear while eliminating incorporation of metal-based detergent-dispersants, Zn-DTP or the like, or reducing amount thereof, and proposed some approaches: for example, Patent literature 1 is referred. However, those approaches have fallen short of detergency and antiwear, and further improvement of performances has been desired.    Patent literature 1: Japanese Patent Application Laid-Open No. 2003-73685